1. Field of the Invention
This invention relates to cross-linkable polyurethane molding compositions.
2. The Prior Art
Essentially linear polyurethane resins resulting from the step-growth reaction of a polyisocyanate and a diol have been employed extensively in the past for fabricating both cellular and non-cellular shaped articles. The underlying step-growth polymerization reaction is particularly adapted for fabricating such articles by the reaction injection molding (RIM) technique. The latter operation has now accordingly become the general molding procedure of choice in the relevant industry because of the substantial inherent economic advantages associated therewith. However, the linear polyurethane resins, in spite of being particularly adapted for RIM processing, have only rather limited end-use industrial applications due to the lack of structural strength properties. The ever expanding use of structural plastic members in the manufacture of automobiles and like assemblies has accordingly prompted efforts to upgrade the mechanical strength properties of the conventional polyurethane resins.
One way to achieve improved properties in this regard is to increase the cross-link density of the resultant polymeric structure. This can be realized in the most straightforward economical manner by employing as part of the conventional diol content a polyol having a hydroxyl functionality in excess of two. Representative recent patents following this tack for upgrading the mechanical strength properties of a typically composed polyurethane resin include U.S. Pat. Nos. 4,260,718; 4,376,834 and 4,385,133. This relevant prior art demonstrates the degree of improvement that can be realized in accordance with this approach. A notable disadvantage of this prior art is only tacitly indicated therein. It is to be accordingly noted that an extended post-cure cycle is required in order to attain the improvement in mechanical strength properties residing in the practice of this prior art. When this disadvantage is considered in context of RIM processing it can be seen how seriously the efficiency of such a molding operation is compromised. It is therefore the object of this invention to obviate the need for post-curing as called for in the practice of the aforementioned prior art.